Classes, Objects, and World-level Methods

1
Classes, Objects, and World-level Methods

• Alice

2
Larger Programs

• As you become more skilled in writing programs,
  you will find that programs quickly increase to
  many, many lines of code.
• Games and other "real world" software
  applications can have thousands, even millions of
  lines of code.

3
Classes, Objects, Methods

• Object-oriented programming uses classes,
  objects, and methods as basic programming
  components.
• These components help to
• organize a large program into small modules
• design and think about an intricate program
• find and remove errors (bugs)

4
In our programs,we have been using

• Classes
• In Alice, classes are predefined as 3D models
• Objects
• An object is an instance of a class.
• Class Frog (Uppercase name)
• Objects frog, frog1, frog2, frog3
• (lowercase names)

5
We have also used

• built-in (predefined) methods
• Examples move, turn to face, say
• World.my first method
• Example
• In the FirstEncounter world, we wrote program
  code where a robot was surprised by an alien.
• All the program code was written in this one
  method, see next slide

6

7
Potential Problem

• The program code just seemed to grow and grow.
• If we continue to write programs this way the
  programs will become longer and more difficult to
  read and think about.

8
Solution

• A solution is to organize the instructions into
  smaller methods.
• A possible storyboard

Do in order surprise spiderRobot and
alienOnWheels surprise each other
investigate spiderRobot gets a closer look at
alienOnWheels react alienOnWheels hides
and spiderRobot sends message
9
Next Step

• The next step is to break down each major task
  into simpler steps.
• Example

surprise Do in order alienOnWheels moves up
alienOnWheels says "Slithy toves?"
spiderRobot's head turns around
10
Stepwise Refinement

• The process of breaking a problem down into large
  tasks and then breaking each task down into
  simpler steps is called stepwise refinement.
• Once the storyboard is completed, we write a
  method for each task.

11
Demo Starting a new method

• First, to associate the new method with the World
• select the World tile in the Object Tree
• select the methods tab in the details area
• click on the "create new method" button

12
Demo

• Ch04Lec1FirstEncounter
• Concepts illustrated in this example world
• surprise is a world-level method because it is
  defined as a method for World and has
  instructions that involve more than one object
  (spiderRobot, alienOnWheels)
• The surprise method is executed by calling
  (invoking) the method .

13
Why?

• Why do we want to write our own methods?
• saves time -- we can call the method again and
  again without reconstructing code
• reduces code size we call the method rather
  than writing the instructions again and again
• allows us to "think at a higher level"
• can think surprise instead of
• The alien moves up and says Slithy toves? and
  then the robot's head turns around. "
• the technical term for "think at a higher level"
  is "abstraction"

14
Assignment

• Read Chapter 4 Section1
• World-level Methods
• How to create them
• How to call them
• When it is appropriate to use them

15
Lab

• Chapter 4 Lab 1